. The goal of this proposed research is to quantitatively determine the scattering and absorption factors of tumor- and nearby normal-tissue volumes in vivo. This high level of quantitation will then enable us to assign physiological meaning to the macroscopic tissue optical properties, thereby clarifying and further improving the specificity of the diffusive wave optical method. The investigation combines several theoretical analysis schemes with a unique pre- and intraoperative protocol wherein photon migration probes are used in the proximity of the tumor tissue volume to measure optical properties; in most cases, other techniques such as ultrasound will also be used as an independent means to characterize the position, size, and shape of the tumor volume. By reducing the experimental uncertainty in the tumor size and position, the primary function of the optical device will be to ascertain tissue optical properties. The information obtained will define the feasi bility of, and lay the foundation for more ambitious low resolution functional imaging instruments and monitors based on photon migration principles. The results will also provide us with new insights about the physiology of tumors such as their oxygen utilization and their vasculature structure.